Along with general-purpose applications of an electrophotographic technology, an image forming apparatus of an electrophotographic system has come into use in various image forming processes. In particular, downsizing of devices for office applications and the like is proceeding. In turn, as concerns the electrophotographic photoreceptor (hereinafter, sometimes referred to as “photoreceptor”), size reduction proceeds and to keep step with speeding-up of the printing speed, out of characteristics of the photoreceptor, quick responsivity, that is, rapid attenuation of the surface potential after imagewise exposure, becomes an important condition. The responsivity corresponds to light attenuation for a given exposure dose and therefore, it also becomes a necessary condition in many cases to elevate the sensitivity. Furthermore, from the standpoint of achieving a high image quality, it is demanded that the electrical characteristics have repetition stability and the photosensitive layer is free from production of a geometric defect or attachment of a material.
Patent Document 1 discloses that a high-sensitive photoreceptor with good repetition stability can be obtained by using a specific fluorene-based compound as the charge transport substance. Patent Document 2 discloses a single layer-type photoreceptor in which a transfer memory is caused to hardly occur by using the above-described fluorene-based compound and a diphenoquinone-based compound. Patent Document 3 discloses that the sensitivity, repetition characteristics and transfer memory are improved by combining the above-described fluorene-based compound and a diamine compound.
Furthermore, along with cost reduction of an electrophotographic apparatus, use of an inexpensive member is expanding. In particular, as concerns the member that comes into contact with the photoreceptor, such as charging roller and developing roller, various additives and the like such as plasticizer are used and therefore, not only contamination of the photoreceptor surface is merely caused but also cracking attributable to an attached material is sometimes generated in the photoreceptor surface. Such a crack sometimes emerges as an image defect and becomes a fatal defect in view of image quality. There are various views as to the cause for a crack, but it is considered that cracking is partly attributed to elution of a photoreceptor component and formation of a void, which are caused by an attached material on the photoreceptor surface, and this is known to be improved by the composition of the photosensitive layer.
With respect to resistance against the above-described cracking, in Patent Document 4, whether cracking of a photosensitive layer after long time storage of a sebum-attached photosensitive layer, which is also called solvent cracking, occurs or not is described including the photosensitive layer compositions disclosed in Patent Documents 1 to 3. Observation results of cracking due to attachment of sebum are not only indicative of the effect of literal finger contact with the photosensitive layer but also applicable to a case where an extraneous material derived from a member such as roller is attached, and these properties are often collectively called crack resistance.
The crack resistance is evaluated in many cases by the presence or absence of cracking when, as described above, sebum is attached and the photoreceptor is stored for a long time. However, in this method, not only individual difference in the evaluation or reproducibility varies greatly but also quantitative determination of the crack resistance is difficult. In addition, the test by attachment of sebum cannot be a good evaluation method as the method for evaluating the resistance to the above-described component that is bled out from a member put into contact with the photoreceptor, because the mechanism of cracking differs or the load on the photoreceptor is often insufficient and the test results are diverged from each other in many cases.